Wet process for phosphoric acid manufacture



lJuly28, 1959 H. svA'NoE r 2,897,053

wm" PRocEss Foa PHosPHoRIc Acro MANUFACTURE Filed Feb. 2e. 1954 ATORNEY WET PROCESS FOR PHOSPHORIC ACID MANUFACTURE Hans Svanoe, Warren, Pa., assiguor to Smithers Wells Corporation, Warren, Pa., a corporation of Maryland Application February 26, '1954, Serial No. 412,833

4 Claims. (Cl. 23-165) This invention is directed to a process for the manufacture of phosphoric acid from phosphate rock and like sources of phosphorus by the so-called wet process and is directed more particularly to controlling the addition of inorganic acids to the reaction mixture to facilitate separation of calcium salt crystals such as calcium sulfate from that mixture.

Present methods for the production of phosphoric acid in the wet way are usually characterized by a preliminary conversion of phosphatic material to crude phosphoric acid by treatment of the raw material with sulfuric acid. The subsequent purification necessary is attended With many difficulties, and such processes are further disadvantageous by reason of the extensive concentration operations necessarily applied to the phosphoric acid to obtain a product suitable for commercial use. Y

Many and various attempts have been made to accomplish rapid settling of calcium sulfate to increase the elhciency of calcium sulfate filtering operations as well as other operations used in the wet process. These attempts have met with but a moderate degree of success because of the inabilityl of prior art investigators to devise a process for precise control of calcium sulfate precipitation and the inadequacy of the filtration equipment available.

A Swiss company, Kunstd'ngen Patent Verwertungs A.G., and a Swedish group, Kemi'ska Patenter, have been working intensively on this problem in conunction with interests in this country, and, although these efforts have improved the process heretofore in use, their efforts have resulted in a process that still requires the use of such elaborate equipment that costs of phosphoric acid and by-products of the process have remained far higher than the costs of the basic raw materials justify.

An object of the present invention is to provide an improved process for the preparation of phosphoric acid from phosphatic material. Another object is to provide such a process wherein the temperature gradient caused by the addition of an inorganic acid to phosphatic materials is reduced to a minimum. Yet another object is to provide an efficient method of forming calcium sulfate crystals of such size and form that they can be rapidly separated lby washing from the mother liquor without caking on the filters. Still another object is to provide means for forming the aforesaid crystals under constant environmental conditions. Other objects and advantages of the invention will hereinafter appear.

The advantages and other features of the invention are many. Some are set forth in this general description of the process, while other features willbe more speciflcally set forth hereinafter with reference to the attached diagrammatic owsheet of the process. In the manufacture of phosphatic fertilizers from phosphate rock in accord With the invention, the rockis prepared by wet grinding in phosphoric acid, and preferably in a part of the produced phosphoric acid liquor. Subsequent to the grinding and solution of the rock by the 2,897,053 Patented July 28, 1959 necessary to operate the process.

ature and concentration of the solution or both.V

The precipitant commonly used is sulfuric acid which gives calcium sulfate as the precipitate. The chemical form of the precipitated calcium sulfate, whether as hemi-hydrate,(CaSO4.%H2O), rgypsum (CaSO4.2I-I2O), or as an anhydrite (CaSO4) is dependent on the temper- In addition, the crystal size and habit are governed, inter alia, by the degree of supersaturation of the solution, the nuclei present in the reaction mixture, the concentration of solution, ratio of nuclei to salt concentration,

time of contact, and mobility of the slurry. By and only by proper selection of not only the chemical form but also the crystal habit is it possible to produce a crystal that can be readily separated from thesupersaturated reaction mixture, and after separation cleanly and eiciently washed free of the mother liquor, which is the concentrated reaction mixture.

vThe invention will be more fully appreciated by reference to the attached diagrammatical flow sheet of the invention in which phosphate rock, pebble phosphate, rock acid slurry and/ or any suitable type of phosphatic material is treated with phosphoric acid and the resulting reaction product further treated with sulfuric acid in accord with these general chemical reactions:

It is, of course, understood that other reactions also take place due to the multiplicity of impurities present in the phosphate rock. The principal Reactions 1 and 2, however, are of primary importance to the process of this invention. The diagrammatical ilowsheet illustrates the simplicity of equipment that is used for the invention. The quantities and reactants given illustrate a preferred operation of the process. In reactor 3 Reaction 1 is conducted. ln crystallizer 15 the suspension and growth of the salt crystals are controlled to give a size and crystal form that can be easily separated and Washed. It should benoted that Reaction 2 is started in a stream of liquor rwithdrawn from the crystallizer 15 and after the addition of the materials returned to the Vcrystallizer 15. By means of these and other suitable equipment, operated as hereinafter described, a highly eicient process for the manufacture off phosphoric acid and byproducts is provided. Y

Reaction l is carried out in the stirred reactor 3 into which ground phosphate rock or any other suitable form of this or similar phosphatic materialv is mixed and dissolved with an excess of phosphoric acid, preferably to give a product in which 35% to 40% by weight of the total P205 present after solution is present as monocalcium phosphate. Solution is facilitated by ecient stirring. Gases produced are discharged through vent 4 to a scrubbing towerV not shown. Sufficient phosphoric acid is added to keepall the monocalcium phosphate in solution, the hold-up time in reactor 3 ybeing ample to insure solution of therock disregarding impurities. l

The product of Reaction l leaves reactor 3 through pipe 5 and is forced by pump 6 into the Hcrystallizer 15 through pipes 7 and 8. Sulfuric acid (of suitable concentration) 'is added to theY solution from supply line 9 as the product of Reaction 2 flowsA through pipe7 wherein Reaction 2 takes place. The calcium sulfate 3 formed is held in solution in the supersaturated state, and the resulting solution is then forced by pump l through pipe 8 into the bottom of the crystallizer l5 from the open'endll of pipe 8. The addition of monocalcium phosphate solutionfrom pipe l2 Vand/ or sulfuric acid from line 9 can be made before or after the solution passes pump 10.

Crystals formed in crystallizer are drawn from a section near the bottom through pipe 13 and separated from the mother liquor on the filter 17, the mother liquor being collected in the receiving tank 18 from which the phosphoric acid product can be delivered lto storage. portion of the phosphoric acid in tank 18 is used to treat the phosphate rock as previously described.

The crystals on filter 17 can be washed with Water and the wash water used for treatment of the phosphate rock in reactor 3 and/or returned to crystallize-r 15.

Means are provided (a vacuum evaporation means 19 with steam ejector 21 and usual auxiliary equipment is illustrated in the ilow sheet, although any other suitable means, such as external or internal cooling, may be used), to cool and preferably to concentrate a portion of the suspension liquid from crystallizer 1S. VThe suspended liquid is drawn from pipe 7 by pump 14 and chemical as well as physical form of the crystals to bc made containing the calcium and sulfate ions.

While the drawing illustrates a preferredV method of cooling and concentrating a portion of the suspensoin recycle, other methods may be used that provide a means of removing the exothermic heat of Reaction 2 from that reaction mixture prior toits introduction into thesuspension in crystallizer l5. Removal of this heat prevents local superheating and resultant formation of very small crystals as well as lundesired forms of calcium sulfate. Y

When Vthe sulfuric acid and the calcium phosphatephosphoric acid solution from Reaction vl are added to the recycled suspension a certain amount of calcium sulfate supersaturation is Vautomatically produced. The method used kfor the combining of these raW materials is an important feature of the invention. By adding the components to the mixture in the proper ratio, the calcium sulfate will not immediately precipitate but will remain dissolved in the circulating stream giving a supersaturated solution. the sulfuric acid be added in slight excess, say 1% to 2% over that stoichiometrically required for Reaction 2. 1 By operating the process as described, a solution is produced that is supersaturated to a slight degree above that maintained in crystallizer 15. This increase in supersaturation is released on the crystals that are suspended in the crystallization chamber l5 preferably main- For this and other reasons it is important that tained at a temperature between b C. and 90 C., and Y benbetween about SQto about 200 gallons per minute perpound lof calcium sulfate produced per minute.

mother liquor to raw materials.

The monocalcium phosphate produced in reactor 3 will reduce the solubility of calcium sulfate from as much as about 12 to 14 grams per liter, which is approximately the concentration of that salt in the aqueous phosphoric acid produced by the process, to as low as l to 2 grams per liter, approximately the solubility after Reaction l. Consequently, in the cyclical processof the invention theV eliicient separation is dependent in no small degree on the passing of the aqueous phosphoric acid solution carrying its calcium sulfate load from a range of high solubility in the crystallizer 15 to a range of low solubility in the reactor 3.

lt is important, furthermore, that sufficient time be il provided for the supersaturation produced by ^the addition of sulfuric acid to be released on the crystals suspended in the mother liquor in the crystallize'r 15, so that when the circulating liquor again arrives 4at the mixing* point in pipe S, a substantial part of the supersatura- Vtion of the solution leaving pipe lll will have been released.' The average retention time of any given crystal within the crystalli'zer l5 should not be less than two hours and is preferably between four and six hours.

The process of the invention produces crystals that can be easily filtered, centrifuged or otherwise separated from the mother liquor. They are larger and more uniform than the calcium sulfate crystals produce'dby the processes in general use today and are substantially free from caking. A`Although the addition of sulfuric acid to the phosphate rock-phosphoric acid solution gives a highly exotrermic reaction, the cooling effect provided on the recycled liquors is such that on return Vto the crystallizer i5 the stream of reaction products from Reaction 2 is substantially at the temperature maintained in crystallizerk which is to a large extent responsible for the improved crystal formation.

cium sulfate containing crystals in pipe 8 by adding the reaction product ofleaction l and sulfuric acid'to the circulating mother liquor in prescribed proportions. These materials should be added in such` proportions to the circulating mother liquor that the crystals will not immediately form by crystallizing from the circulating liquor but the salt will remain dissolved therein to give a supersaturated solution. The ratio can vary through fairly wide limits but preferably is maintained between 1000 to 1500 parts by weight of mother liquor per part of calcium sulfate produced by Reaction 2.v By adding sulfuric acid in amounts to react stoichiometrically with the monocalcium phosphate produced by Reaction 1, and to the extent stated supra, the recycled portion of mother liquor at any one time will have an increaseV in supersaturation to a relatively slight degree, generally between 0.5, to 5.0 grams per liter over the grams per liter Yat saturation in the circulating liquor under the temperature of therliquor after the addition of the sulfuric acid or other anion. Production of a slightly supersaturated solution in the recycling mother liquor is releasedV to a sutilcient degree inthe crystallizer l5 to give excellent crystal growth and the desired crystal form. VThe highly 'exothermic Reaction 2 can therefore be carried out without disturbingy the delicately balanced `preferred crystallization conditions of the suspension of crystals in crystai- Vlizer 15.*

The most satisfactory method of removing the heat from the reaction mixtures of Reactions 1 and 2, which is close to 1000 B.t.u. per pound of P205 produced as phosphoric acid, is illustrated in the drawing. A separate liquor stream is removed from `the crystallizer at any suitable point but before the addition of sulfuric acid and monocalcium phosphate solution to the recycled liquor. This liquor is pumped by pump 14 into vaporizer 19, where the reaction heat is removed by vaporizing an amount of water the heat of vaporization of which is equivalent to the -amount of reaction heat. Due to the fact that calcium sulfate has a positive solubility increase with temperature in this liquor, it is important that the temperature decrease in the separated liquor is such that the liquor is maintained in the metastable zofne wherein excessive formation of nuclei is prevented. Hence fewer and larger crystals are formed. With operation in the crystallizer at temperatures between 65 C. and 85 C., reaction heat is removed by maintaining temperature drop of the liquid to and from vaporizer 19 to between 2 C. to 5 C. and preferably between about 3 C. to 4 C., the cooled liquor as shown in the drawing being returned tothe crystallizer 15. By so operating, the degree of supersaturation produced by mixing of the raw materials to produce Reaction 2 as Well as the degree of supersaturation produced in the cooling cycle is of the same magnitude and in the region where excess nucleation is prevented.

One embodiment of the invention comprises feeding with an excess of phosphoric acid in accord with Reaction 1, 12,160 lbs/hr. of tricalcium phosphate calculated as tricalcium phosphate in phosphate rock to reactor 3 maintained at a temperature of 75 to 90 C. From reactor 3, line 5 carries (per 100 lbs. by weight of P205 as monocalcium phosphate) 155 lbs. P205 as carrier acid and 520 lbs. of water which are introduced into pipe 7 ltogether with 11,500 lbs./ hr. of sulfuric acid diluted With 235 lbs/hr. of water and recycled slurry from crystallizer 15, the crystallizer being maintained at a temperature of 65 to 80 C. The slurry removed from crystallizer 15 through line 13 contains 15 to 35 by weight of gypsum which is separated in filter 17 at the rate of 20,200 lbs/hr. calculated as CaS04.2H2O. The ltrate from filter 17 is collected in tank 18 and the produced phosphoric acid is discharged from this tank at the rate of 5,600 lbs./hr. of P205 with 13,070 lbs./hr. of water.

The coarseness anduniformity of crystals produced in the crystallizer can be controlled within fairly Wide limits such that rapid ltration, washing and separation from lter presses and centrifuges is a simple, inexpensive operation. The consistency of the slurry conducted to the lter can be between 15 to 30% crystal by Weight. The phosphoric acid containing 25 to 35% P205 or even higher is conducted to other process operations and the necessary acid returned to the reactor.

Whereas, the process of the invention has been described primarily for Reaction 2 and with sulfuric acid as the acid reactant, mixtures of nitric and sulfuric acid may be used. Moreover, the calcium ion can be precipitated by some soluble sulfate salt such, for example, as ammonium-, sodiumor potassium sulfate and the same is the case with the monocalcium phosphate produced in the reactor which may be formed in accord with the processes of these reactions:

in which M may be Na or K.

However, the process of this invention whereby supersaturation caused by the acid addition is eiected outside the major crystallization zone makes it possible to carry out such processes more continuously and cfiiciently than has heretofore been possible.

I claim:

1. In a process for the preparation of phosphoric acid from phosphate containing rock `and sulfuric acid the steps which comprise reacting phosphate rock with phosphoric acid substantially in accord with this reaction:

thereafter reacting the resulting reaction mixture with sulfuric acid substantially in accord with this reaction:

forming a Isuspension of growing crystals of calcium sulfate in motherY liquor from the reaction mixtures of Reactions l and 2, withdrawing a stream of mother liquor from the suspension, adding sulfuric acid and the reaction product of Reaction 1 to the withdrawn stream in such proportions that crystals of calcium sulfate will not immediately form by crystallizing from the stream, withdrawing a separate stream of mother liquor from the suspension, removing from the separate stream by vapor-ization Ian amount of water the heat of vaporization of which is equivalent to the amount of reaction heat of said Reactions 1 and 2, combining the thus treated streams and returning the combined streams to said suspension.

2. The process of claim 1 in which the degree of supersaturation in the Awithdrawn mother lliquor `after the addition of sulfuric yacid is between 0.5 and 5.0 grams per liter of calcium sulfate 4greater than the supersaturation of the calcium sulfate in the suspension.

3. The process of claim 1 in which the suspension is maintained at a temperature between C. and 85 C.

4. The process of claim 1 in which a portion of the withdrawn mother liquor is subjected to vacuum evaporation and the thus cooled solution returned to the suspension with the sulfuric `acid-phosphoric yacid-phosphate rock reaction mixture at substantially the temperature of the suspension.

References Cited in the le of this patent UNITED STATES PATENTS Re. 19,045 Larsson Jan. 16, 1934 1,667,549 Hechenbleikner Apr. 2A, 1928 1,836,694 Wadsted Dec. 15, 1931 1,944,048 Walker et al. Ian. 16, 1934 2,049,032 Weber et al. July 28. 1936 2,233,956 Moore Mar. 4, 1941 2,531,977 Hammaren et al. Nov. 28, 1950 

1. IN A PROCESS FOR THE PREPARATION OF PHOSPHORIC ACID FROM PHOSPHATE CONTAINING ROCK AND SULFURIC ACID THE STEPS WHICH COMPRISE REACTING PHOSPHATE ROCK WITH PHOSPHORIC ACID SUBSTANTIALLY IN ACCORD WITH THIS REACTION: (1) CA3(PO4)+4H3PO4=3CAH4(PO4)2 THEREAFTER REACTING THE RESULTING REACTION MIXTURE WITH SULFURIC ACID SUBSTANTIALLY IN ACCORD WITH THIS REACTION: (2) 3CAH4(PO4)2+3H2SO4=3CASO4+6H3PO4 FORMING A SUSPENSION OF GROWING CRYSTALS OF CALCIUM SULFATE IN MOTHER LIQUOR FROM THE REACTION MIXTURES OF REACTIONS 1 AND 2, WITHDRAWING A STREAM OF MOTHER LIQOUR FROM THE SUSPENSION, ADDING SULFURIC ACID AND THE REACTION PRODUCT OF REACTION 1 TO THE WITHDRAWN STREAM IN SUCH PROPORTION THAT CRYSTALS OF CALCIUM SULFATE WILL NOT IMMEDIATELY FORM BY CRYSTALLIZING FROM THE STREAM, WITHDRAWING A SEPARATE OF MOTHER LIQUOR FROM THE SUSPENSION, REMOVING FROM THE HSEPARATE STREAM BY VAPORIZATION AN AMOUNT OF WATER THE HEAT OF VAPORRIZATION OF WHICH IS EQUIVALENT TO THE AMOUNT OF REACTION HEAT OF SAID REACTION 1 AND 2, COMBINING THE THUS TREATED STREAMS AND RETURNING THE COMBINED STREAMS TO SAID SUSPENSION. 